Re: [time-nuts] GNSS Disciplined Clock

[Mike Cook]

> Le 27 mai 2017 à 21:26, Ebrahim Roghanizad  a écrit :
> 
> Dear Chris
> 
> As far as I know, atmospheric effect can not be compensated by looking at
> satellites from all over the sky and averaging, since it does not have a
> random nature, rather it introduces bias to the solution. For example, if
> atmospheric effect is not removed, one can not get a relative position
> accuracy of sub-meter in long distances even by employing the method of
> RTK. All I said here is about position. Now, I would like to know about the
> output of time in this condition. What is the utmost reachable accuracy for
> a timing output from a GNSS receiver? I do not mean the precision that
> reflects the noise behavior. I think that the best result is obtained when
> the receiver supports dual frequency in order be able to deal with
> ionospheric delay. Am I right? In that case, is there any GNSS receiver
> with this ability?
> 

 I don’t know of any and over time have been looking for one . I guess there is 
no market for a pure GNSS solution. Current L1 only timing receivers can offer 
down to +/-6ns accuracy with quantization error data allowing correction of 
their PPS output down to the stability of the GPS signal. Using that data and 
available cheap delay line chips the 1PPS accuracy deliverable can be reduced 
to around +/-2-3ns .
It appears to be cheaper to use just the L1 derived time to lock better 
oscillators for better precision than that. 


> Thanks a lot
> 
> On Thu, May 25, 2017 at 10:01 PM, Chris Albertson > wrote:
> 
>> The long term stability of GPS is very good.  Some one here will point
>> out exactly how one measures it.   But roughly when speaking of
>> accuracy you always need to specify a time interval. For example
>> if the 1PPS is "off" by 15ns that is not bad and yes there are much
>> better systems if you need to measure time intervals on the order of
>> one second.   But if the signal is "off" by 15 ns over 100,000 seconds
>> that is well, 100,000 time better.
>> 
>> This is a basic reference and for some specialized end use case you
>> might couple it with other equipment.  Many of the concerns you had,
>> such as effects of the atmosphere get averaged out because the unit is
>> looking at satellites from all over the sky.  (averaging over space)
>> And you do git better results with better antenna locations that are
>> away from multi-path and have a 360 degree view of the horizon.  But
>> notice the unit has an temperature stabilized crystal oscillator that
>> is stable over many seconds. an is much more stable in the short term
>> then is a GPS receiver.  Trimble uses this crystal to average over
>> time
>> 
>> You also have to ask where is the tine data going to be used.  Are you
>> synchronizing a computer's internal clock or trying to measure the
>> frequency of a microwave transmitter
>> 
>> SO it falls back to the old thing about there being no "better" only
>> better for a specific use case.
>> 
>> Some of use were lucky enough to buy Trimble Thunderbolts, a previous
>> version of this unit when they were on eBay for $100 each.   For those
>> without 5 digits budget they ar pretty much the Gold Standard.  I have
>> mine installed with a good filtered DC power supply and an outdoor
>> antenna on mast well above the roofs of surrounding buildings. I
>> get long term stability of about one part in 10E13.   Yes 13 digits
>> over long periods.   (I think?)  It is really hard to know because my
>> measurement system is a little circular referenced
>> 
>> On Thu, May 25, 2017 at 6:27 AM, Ebrahim Roghanizad
>>  wrote:
>>> Dear members
>>> 
>>> I am a new amateur member in your group. Maybe my question has been
>> asked.
>>> Recently I found Trimble Mini-T GG, whose data sheet is attached, as a
>> good
>>> GNSS disciplined time reference. I would like to know if there exists a
>>> more accurate one, since it does not employ dual frequencies to
>> compensate
>>> ionospheric delay, though it handles both GPS and GLONASS. Besides, could
>>> anyone guide me about the presented accuracy in the datasheet? There, it
>> is
>>> stated that "When operating in Over Determined Timing Mode, the accuracy
>> of
>>> pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it
>> mean
>>> that it includes both bias and the noise? In other words, is it true to
>> say
>>> that "The time-synchronization error between two of them with a long
>>> distance is less than 2*15 ns"?
>>> 
>>> Best Regards
>>> 
>>> ___
>>> time-nuts mailing list -- time-nuts@febo.com
>>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>> mailman/listinfo/time-nuts
>>> and follow the instructions there.
>> 
>> 
>> 
>> --
>> 
>> Chris Albertson
>> Redondo Beach, California
>> ___
>> time-nuts mailing list -- time-nuts@febo.com
>> To unsubscribe, go to 

Re: [time-nuts] GNSS Disciplined Clock

[Bob kb8tq]

Hi

Second to second jitter *could* be  another interpretation of the 15 ns. That 
would be a pretty
loose number. Modern stuff after sawtooth correction gets down to < 1 ns on 
that basis. Since it’s 
a GPSDO, I would guess it’s under 1 ns (1x10^-9) at one second. 

Correcting for timing bias to get back to UTC is an involved process. The 
“easy” way to do it
is to run a dual frequency receiver and run the RINEX data through post 
processing. That only helps
you for “past history”. It is the most common way to do it. The same dual 
frequency process also 
takes care of the X,Y,Z stuff. 

Bottom line - with a single frequency device, you don’t have a lot of options 
for eliminating the bias 
issues.

Bob

> On May 27, 2017, at 2:56 PM, Ebrahim Roghanizad  
> wrote:
> 
> Dear Bob
> 
> As I understand from your response, 15 ns reflects the jitter, i.e. the
> time difference between successive PPSs is not exactly 1 second and its
> standard deviation (one sigma) is 15 ns. In that case, "precision" is more
> appropriate than "accuracy" to be used in the datasheet. Moreover, how do
> you infer horizontal and vertical position *accuracy*? As well, could you
> please guide me to find its timing bias with respect to GNSS/UTC?
> 
> Thanks a lot
> 
> On Thu, May 25, 2017 at 8:26 PM, Bob kb8tq  wrote:
> 
>> Hi
>> 
>> I don’t speak for Trimble and their interpretation of all this may be a
>> bit unique. Normally
>> what the 15 ns means is the time out of a simulator compared to the 1 pps
>> out of the module.
>> Put another way, it’s just a measure of the receiver. It does not include
>> any ionosphere / troposphere
>> issues. It assumes a perfect estimate of the location (no bias from
>> antenna multipath). IT also
>> does not take into account any delay in the antenna or coax to the
>> antenna. Time errors between
>> Glonass and GPS are not included (bad broadcast offset estimate etc).
>> Finally there is the
>> fairly important qualifier of “one sigma” on the 15 ns number.
>> 
>> All that said, two devices with the same antennas, same cables, close to
>> each other, looking at the
>> same sats, using the same systems, … should track pretty well.
>> 
>> Bob
>> 
>>> On May 25, 2017, at 9:27 AM, Ebrahim Roghanizad 
>> wrote:
>>> 
>>> Dear members
>>> 
>>> I am a new amateur member in your group. Maybe my question has been
>> asked.
>>> Recently I found Trimble Mini-T GG, whose data sheet is attached, as a
>> good
>>> GNSS disciplined time reference. I would like to know if there exists a
>>> more accurate one, since it does not employ dual frequencies to
>> compensate
>>> ionospheric delay, though it handles both GPS and GLONASS. Besides, could
>>> anyone guide me about the presented accuracy in the datasheet? There, it
>> is
>>> stated that "When operating in Over Determined Timing Mode, the accuracy
>> of
>>> pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it
>> mean
>>> that it includes both bias and the noise? In other words, is it true to
>> say
>>> that "The time-synchronization error between two of them with a long
>>> distance is less than 2*15 ns"?
>>> 
>>> Best Regards
>>> ___
>>> time-nuts mailing list -- time-nuts@febo.com
>>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>> mailman/listinfo/time-nuts
>>> and follow the instructions there.
>> 
>> ___
>> time-nuts mailing list -- time-nuts@febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>> mailman/listinfo/time-nuts
>> and follow the instructions there.
>> 
> ___
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> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

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Re: [time-nuts] GNSS Disciplined Clock

[Ebrahim Roghanizad]

Dear Chris

As far as I know, atmospheric effect can not be compensated by looking at
satellites from all over the sky and averaging, since it does not have a
random nature, rather it introduces bias to the solution. For example, if
atmospheric effect is not removed, one can not get a relative position
accuracy of sub-meter in long distances even by employing the method of
RTK. All I said here is about position. Now, I would like to know about the
output of time in this condition. What is the utmost reachable accuracy for
a timing output from a GNSS receiver? I do not mean the precision that
reflects the noise behavior. I think that the best result is obtained when
the receiver supports dual frequency in order be able to deal with
ionospheric delay. Am I right? In that case, is there any GNSS receiver
with this ability?

Thanks a lot

On Thu, May 25, 2017 at 10:01 PM, Chris Albertson  wrote:

> The long term stability of GPS is very good.  Some one here will point
> out exactly how one measures it.   But roughly when speaking of
> accuracy you always need to specify a time interval. For example
> if the 1PPS is "off" by 15ns that is not bad and yes there are much
> better systems if you need to measure time intervals on the order of
> one second.   But if the signal is "off" by 15 ns over 100,000 seconds
> that is well, 100,000 time better.
>
> This is a basic reference and for some specialized end use case you
> might couple it with other equipment.  Many of the concerns you had,
> such as effects of the atmosphere get averaged out because the unit is
> looking at satellites from all over the sky.  (averaging over space)
> And you do git better results with better antenna locations that are
> away from multi-path and have a 360 degree view of the horizon.  But
> notice the unit has an temperature stabilized crystal oscillator that
> is stable over many seconds. an is much more stable in the short term
> then is a GPS receiver.  Trimble uses this crystal to average over
> time
>
> You also have to ask where is the tine data going to be used.  Are you
> synchronizing a computer's internal clock or trying to measure the
> frequency of a microwave transmitter
>
> SO it falls back to the old thing about there being no "better" only
> better for a specific use case.
>
> Some of use were lucky enough to buy Trimble Thunderbolts, a previous
> version of this unit when they were on eBay for $100 each.   For those
> without 5 digits budget they ar pretty much the Gold Standard.  I have
> mine installed with a good filtered DC power supply and an outdoor
> antenna on mast well above the roofs of surrounding buildings. I
> get long term stability of about one part in 10E13.   Yes 13 digits
> over long periods.   (I think?)  It is really hard to know because my
> measurement system is a little circular referenced
>
> On Thu, May 25, 2017 at 6:27 AM, Ebrahim Roghanizad
>  wrote:
> > Dear members
> >
> > I am a new amateur member in your group. Maybe my question has been
> asked.
> > Recently I found Trimble Mini-T GG, whose data sheet is attached, as a
> good
> > GNSS disciplined time reference. I would like to know if there exists a
> > more accurate one, since it does not employ dual frequencies to
> compensate
> > ionospheric delay, though it handles both GPS and GLONASS. Besides, could
> > anyone guide me about the presented accuracy in the datasheet? There, it
> is
> > stated that "When operating in Over Determined Timing Mode, the accuracy
> of
> > pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it
> mean
> > that it includes both bias and the noise? In other words, is it true to
> say
> > that "The time-synchronization error between two of them with a long
> > distance is less than 2*15 ns"?
> >
> > Best Regards
> >
> > ___
> > time-nuts mailing list -- time-nuts@febo.com
> > To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> > and follow the instructions there.
>
>
>
> --
>
> Chris Albertson
> Redondo Beach, California
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> and follow the instructions there.
>
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Re: [time-nuts] GNSS Disciplined Clock

[Ebrahim Roghanizad]

Dear Bob

As I understand from your response, 15 ns reflects the jitter, i.e. the
time difference between successive PPSs is not exactly 1 second and its
standard deviation (one sigma) is 15 ns. In that case, "precision" is more
appropriate than "accuracy" to be used in the datasheet. Moreover, how do
you infer horizontal and vertical position *accuracy*? As well, could you
please guide me to find its timing bias with respect to GNSS/UTC?

Thanks a lot

On Thu, May 25, 2017 at 8:26 PM, Bob kb8tq  wrote:

> Hi
>
> I don’t speak for Trimble and their interpretation of all this may be a
> bit unique. Normally
> what the 15 ns means is the time out of a simulator compared to the 1 pps
> out of the module.
> Put another way, it’s just a measure of the receiver. It does not include
> any ionosphere / troposphere
> issues. It assumes a perfect estimate of the location (no bias from
> antenna multipath). IT also
> does not take into account any delay in the antenna or coax to the
> antenna. Time errors between
> Glonass and GPS are not included (bad broadcast offset estimate etc).
> Finally there is the
> fairly important qualifier of “one sigma” on the 15 ns number.
>
> All that said, two devices with the same antennas, same cables, close to
> each other, looking at the
> same sats, using the same systems, … should track pretty well.
>
> Bob
>
> > On May 25, 2017, at 9:27 AM, Ebrahim Roghanizad 
> wrote:
> >
> > Dear members
> >
> > I am a new amateur member in your group. Maybe my question has been
> asked.
> > Recently I found Trimble Mini-T GG, whose data sheet is attached, as a
> good
> > GNSS disciplined time reference. I would like to know if there exists a
> > more accurate one, since it does not employ dual frequencies to
> compensate
> > ionospheric delay, though it handles both GPS and GLONASS. Besides, could
> > anyone guide me about the presented accuracy in the datasheet? There, it
> is
> > stated that "When operating in Over Determined Timing Mode, the accuracy
> of
> > pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it
> mean
> > that it includes both bias and the noise? In other words, is it true to
> say
> > that "The time-synchronization error between two of them with a long
> > distance is less than 2*15 ns"?
> >
> > Best Regards
> > ___
> > time-nuts mailing list -- time-nuts@febo.com
> > To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> > and follow the instructions there.
>
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> and follow the instructions there.
>
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Re: [time-nuts] GNSS Disciplined Clock

[Tom Van Baak]

> The long term stability of GPS is very good.  Some one here will point
> out exactly how one measures it.   But roughly when speaking of
> accuracy you always need to specify a time interval. For example
> if the 1PPS is "off" by 15ns that is not bad and yes there are much
> better systems if you need to measure time intervals on the order of
> one second.   But if the signal is "off" by 15 ns over 100,000 seconds
> that is well, 100,000 time better.

Chris,

It's not 100,000 times better; it's not better at all.

If you are looking for *timing accuracy* then 15 ns is 15 ns. Doesn't matter if 
it right now, an hour from now, or tomorrow. It's an error, plain and simple. 
It doesn't get significantly better or worse over time. It is often quoted as 
an rms statistic on GPS receiver 1PPS specs. You can measure over a few 
minutes, or a few hours, or a few days -- you'll get approximately the same rms 
timing error.

What you're probably thinking of is *long-term average frequency accuracy* -- 
and then, yes, a bounded error like 15 ns rms looks less and less like a 
problem as you average longer and longer. But this does not mean the "GPSDO is 
getting better over time". All it means is the parameter you chose to measure 
(average frequency) happens to have elapsed time in the denominator, so of 
course the number gets lower as elapsed time goes on. But nothing tangible is 
getting "better" over time.

/tvb

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Re: [time-nuts] GNSS Disciplined Clock

[Chris Albertson]

The long term stability of GPS is very good.  Some one here will point
out exactly how one measures it.   But roughly when speaking of
accuracy you always need to specify a time interval. For example
if the 1PPS is "off" by 15ns that is not bad and yes there are much
better systems if you need to measure time intervals on the order of
one second.   But if the signal is "off" by 15 ns over 100,000 seconds
that is well, 100,000 time better.

This is a basic reference and for some specialized end use case you
might couple it with other equipment.  Many of the concerns you had,
such as effects of the atmosphere get averaged out because the unit is
looking at satellites from all over the sky.  (averaging over space)
And you do git better results with better antenna locations that are
away from multi-path and have a 360 degree view of the horizon.  But
notice the unit has an temperature stabilized crystal oscillator that
is stable over many seconds. an is much more stable in the short term
then is a GPS receiver.  Trimble uses this crystal to average over
time

You also have to ask where is the tine data going to be used.  Are you
synchronizing a computer's internal clock or trying to measure the
frequency of a microwave transmitter

SO it falls back to the old thing about there being no "better" only
better for a specific use case.

Some of use were lucky enough to buy Trimble Thunderbolts, a previous
version of this unit when they were on eBay for $100 each.   For those
without 5 digits budget they ar pretty much the Gold Standard.  I have
mine installed with a good filtered DC power supply and an outdoor
antenna on mast well above the roofs of surrounding buildings. I
get long term stability of about one part in 10E13.   Yes 13 digits
over long periods.   (I think?)  It is really hard to know because my
measurement system is a little circular referenced

On Thu, May 25, 2017 at 6:27 AM, Ebrahim Roghanizad
 wrote:
> Dear members
>
> I am a new amateur member in your group. Maybe my question has been asked.
> Recently I found Trimble Mini-T GG, whose data sheet is attached, as a good
> GNSS disciplined time reference. I would like to know if there exists a
> more accurate one, since it does not employ dual frequencies to compensate
> ionospheric delay, though it handles both GPS and GLONASS. Besides, could
> anyone guide me about the presented accuracy in the datasheet? There, it is
> stated that "When operating in Over Determined Timing Mode, the accuracy of
> pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it mean
> that it includes both bias and the noise? In other words, is it true to say
> that "The time-synchronization error between two of them with a long
> distance is less than 2*15 ns"?
>
> Best Regards
>
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.



-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] GNSS Disciplined Clock

[Bob kb8tq]

Hi

I don’t speak for Trimble and their interpretation of all this may be a bit 
unique. Normally 
what the 15 ns means is the time out of a simulator compared to the 1 pps out 
of the module. 
Put another way, it’s just a measure of the receiver. It does not include any 
ionosphere / troposphere 
issues. It assumes a perfect estimate of the location (no bias from antenna 
multipath). IT also 
does not take into account any delay in the antenna or coax to the antenna. 
Time errors between
Glonass and GPS are not included (bad broadcast offset estimate etc).  Finally 
there is the
fairly important qualifier of “one sigma” on the 15 ns number. 

All that said, two devices with the same antennas, same cables, close to each 
other, looking at the
same sats, using the same systems, … should track pretty well. 

Bob

> On May 25, 2017, at 9:27 AM, Ebrahim Roghanizad  
> wrote:
> 
> Dear members
> 
> I am a new amateur member in your group. Maybe my question has been asked.
> Recently I found Trimble Mini-T GG, whose data sheet is attached, as a good
> GNSS disciplined time reference. I would like to know if there exists a
> more accurate one, since it does not employ dual frequencies to compensate
> ionospheric delay, though it handles both GPS and GLONASS. Besides, could
> anyone guide me about the presented accuracy in the datasheet? There, it is
> stated that "When operating in Over Determined Timing Mode, the accuracy of
> pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it mean
> that it includes both bias and the noise? In other words, is it true to say
> that "The time-synchronization error between two of them with a long
> distance is less than 2*15 ns"?
> 
> Best Regards
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

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